1. Field of the Invention
The present invention relates to a lens-fitted photo film unit having a exposure control mechanism by stop changeover, more particularly, the present invention relates to a lens-fitted photo film unit preventing deterioration of image quality due to change of the stop position.
2. Background Arts
A variety of lens-fitted photo film units, having a pre-loaded photo film and photographic mechanisms like a photographic lens and a shutter mechanism, are manufactured and sold. In order to reduce the manufacturing cost, the lens-fitted photo film unit, hereinafter called simply film unit, is comprised as simple as possible. For instance, a lens holder having a fixed stop opening and a struck-type shutter blade are applied to the film unit. The values of f-number and shutter speed are fixed, and the photographing condition is also fixed.
Therefore, under a severe photographing condition, there is a case where an exposure amount is out of the latitude. In under-exposed condition, a printed image is blurred and low-contrasted, and the graininess becomes worse. In over-exposed condition, on the other hand, it requires much time to expose to a photographic paper in the printing operation.
Recently, since an inexpensive camera like a film unit is required to improve its performance, some mechanisms for changing the photographing condition is considered. For instance, a stop control mechanism according to subject brightness, and a mechanism for changing photographic lenses with different focal lengths according to the subject distance. Since a complex or an expensive mechanism is not be applicable to a cheap camera like a film unit, exposure changeover mechanism by means of moving a stop plate with a stop-down opening is applied to the film unit.
For instance, as illustrated in FIGS. 15A and 15B, when a shutter blade 103 is disposed at the rear of a lens barrel 102 having a pair of photographic lenses 100 and 101, and inserting and retracting a stop plate 104 having a stop-down opening between photographic lenses 100 and 101, a problem described below occurs, because the stop openings are located at the different positions on the optical axis.
As illustrated in FIGS. 15A and 16A, when the stop plate 104 is in the retracted position, fluxes of incident light focusing on an edge of a photo film 105 are overlapped in the shutter blade 103. As illustrated in FIGS. 15B and 16B, on the other hand, when the stop plate 104 is in the inserted position, each flux is separated at the shutter blade 103.
A shutter mechanism applied to film unit, as illustrated in FIG. 17, an exposure is done when a shutter blade is struck and it moves in rotational fashion clockwise. Among the five fluxes of incident light irradiated on the shutter blade, shown with hatched lines in FIG. 17, the time when the left lower flux is cut by the shutter blade is longer than that of the upper right one. The exposure on a photo film by the lower left flux is less than that by the upper right one. Therefore, when fluxes are separated on a shutter blade due to inserting a stop-down opening, exposure unevenness is likely to be conspicuous.
To solve this problem, it is preferable to dispose a shutter blade moveable along the optical axis, and to move the shutter blade when the stop plate is inserted or retracted. However, it is difficult to move the position of the shutter blade, because the shutter blade is required to keep a film chamber dark and it is a member rendering by shutter release operation.
As illustrated in FIGS. 18A and 18B, in changing over photographic lenses with different focal lengths, same problem described above occurs because the optimum positions of stop openings are different. When a wide-angle lens 106 is used, as illustrated in FIG. 18A, fluxes focused on an edge of a photo film 105 are overlapped in the shutter blade 103. On the other hand, when a telephoto lens 107 is used, as illustrated in FIG. 18B, fluxes are separated in the shutter blade 103. For the same reason described above, exposure unevenness is likely to occur.
It is necessary for reducing the exposure unevenness to set the optimum positions equal between in the wide-angle state and the telescopic state, devising powers and positions of photographic lenses. It makes difficult to design the photographic lenses freely.
A photographic lens used in a film unit is designed so that the spherical aberration is corrected somewhat insufficiently to keep a balance of image quality between a peripheral area and a central area. However, because of the spherical aberration, the object distance is different when the exposure opening is inserted on the optical axis, to cause the shift of focus. In that case, it is hard to improve optical performance at any distance by inserting the stop-down opening.
Besides, a photographic lens used in the film unit is required to provide at a low cost, the chromatic aberration is corrected insufficiently. So, there is a case in which only the fluxes with large chromatic aberration can pass through the stop-down opening. In that case, it is hard to improve optical performance by inserting a stop-down opening.
In view of the foregoing, an object of the present invention is to provide a film unit that is able to reduce the effect of exposure unevenness by a stop changeover.
Another object of the present invention is to provide a film unit that is able to reduce the shift of the focus even at the time of a stop changeover.
To achieve the above object, a film unit is comprised of at least one photographic lens for forming an frame on a photo film, and a stop changer disposed on an optical axis of the photographic lens for controlling an amount of subject light, the stop changer being capable of changing over at least at a first position and a second position, making an f-number of the photographic lens f1 when at the first position, making the f-number of the photographic lens f2 when at the second position, wherein f2 greater than f1;
the lens-fitted photo film unit satisfying the following conditions:
|log2(L/100)| less than 1.9
0 less than Exe2x89xa62xe2x88x920.91xc3x97|log2(L/100)|
wherein L(%) represents a proportion brightness in a marginal portion of the frame to brightness in a central portion of the frame at the time when the stop changer is at the second position, and E(EV) represents a difference between the maximum exposure amount and the minimum exposure amount in the marginal portion of the frame.
In the preferable embodiment of the present invention, the film unit is comprised of a photographic lens with two lens elements, and a exposure opening formed inside an unit body, and a stop plate having a stop-down opening which is smaller than the stop opening. The stop plate is movable into a position between the two lens elements. Since the section of incident light with large chromatic aberration is cut by the stop plate, the image quality improves.
According to the present invention, an exposure unevenness has a tendency not to be conspicuous when a marginal brightness is large, even if the exposure difference is large. Therefore, to determine the permissible range of marginal brightness for reducing an effect of exposure unevenness, it is possible to dispose the photographic lens and the shutter blade freely. It gets easier to design the photographic lens.
In the other preferable embodiment, a film unit comprises at least one photographic lens for forming an frame on a photo film, and a stop changer disposed on an optical axis of the photographic lens for controlling an amount of subject light, the stop changer being capable of changing over at least at a first position and a second position, making an f-number of the photographic lens f1 when at the first position, making the f-number of the photographic lens f2 when at the second position, wherein f2 greater than f1;
the lens-fitted photo film unit satisfying the following condition:
xcex4xxc2x7f1/F less than 0.06
wherein F(mm) represents paraxial focal length of the photographic lens, xcex4x(mm) represents a shift of peak positions in MTF curves upon changing over the stop changer, satisfying the following condition:
u=20xc3x9721.63/DL
wherein u(lines/mm) represents a spatial frequency of the MTF curves, DL(mm) represents half a length of a diagonal line of the frame.
By determining the change of peak positions in MTF curves in a certain range, it is possible to reduce the fluctuation of focusing distance, and to improve optical performance at any distance by inserting the stop-down opening.